Evaluating runoff simulations from the Community Land Model 4.0 using observations from flux towers and a mountainous watershed

Previous studies using the Community Land Model (CLM) focused on simulating land-atmosphere interactions and water balance on continental to global scales, with limited attention paid to its capability for hydrologic simulations at watershed or regional scales. This study evaluates the performance of CLM 4.0 (CLM4) for hydrologic simulations and explores possible directions of improvement. Specifically, it is found that CLM4 tends to produce unrealistically large temporal variations of runoff for applications at a mountainous catchment in the northwest United States, where subsurface runoff is dominant, as well as at a few flux tower sites spanning a wide range of climate and site conditions in the United States. Runoff simulations from CLM4 can be improved by (1) increasing spatial resolution of the land surface representations and (2) calibrating model parameter values. We also demonstrate that runoff simulations may be improved by implementing alternative runoff generation schemes such as those from the variable infiltration capacity (VIC) model or the TOPMODEL formulations with a more general power law-based transmissivity profile, which will be explored in future studies. This study also highlights the importance of evaluating both energy and water fluxes in the application of land surface models across multiple scales.